Major depressive illness is alleviated by chronic, but not acute, administration of antidepressant drugs and electroconvulsive seizures (ECS). While the mechanism of action of these widely used treatments is not clearly understood, the delay in therapeutic efficacy has led to the hypothesis that some adaptive alteration in neuronal function must occur over the course of treatment. Preclinical and clinical studies support the hypothesis that the norepinephrine (NE) and serotonin (5-HT) neurotransmitter systems are involved in the therapeutic action of antidepressant treatments, but regulation of neurotransmitter levels and/or receptors cannot fully account for such actions. This is not surprising, since different types of antidepressants exert different effects on NE and 5-HT neurotransmitter systems. An updated hypothesis is that the actions of antidepressant treatments are mediated by adaptations of postreceptor, intracellular sites and regulation of gene expression. Our preliminary studies demonstrate that chronic, but not acute, administration of antidepressants increases levels of cAMP- dependent protein kinase (PKA) enzyme activity in particulate fractions, and levels of cAMP response element binding protein (CREB) immunoreactivity in rat frontal cortex. We have also found that antidepressant treatments increase levels of brain derived neurotrophic factor (BDNF) mRNA and its receptor, trkB, in frontal cortex and hippocampus, and that induction of BDNF is blocked by local infusion of CREB antisense, but not sense, oligonucleotides, providing a functional link between the PKA-CREB cascade and expression of BDNF. Based on these results, we hypothesize that antidepressant treatments activate the PKA- CREB cascade and increase expression of BDNF. To test this hypothesis, we will determine the influence of different types of antidepressant treatments, including electroconvulsive seizure, monoamine oxidase inhibitors, selective NE and 5-HT re-uptake inhibitors, and several atypical antidepressants on levels of PKA and CREB and expression of BDNF and trkB mRNA in rat limbic brain regions. The relevance and significance of these postreceptor adaptations will be verified by several criteria, including analysis of pharmacological specificity, determined by examination of nonantidepressant psychotropic drug treatments (haloperidol, cocaine, diazepam, and morphine), selective dose responses, time course, and regional specificity. PKA will be analyzed by standard enzyme assays and Western blot analysis, CREB immunoreactivity, both phosphorylated and dephosphorylated forms, will be determined by Western blot, CREB function will be determined by CRE gel shift analysis, and BDNF and trkB mRNA will be determined by RNase protection, northern blot, and in situ hybridization. Activation of the PKA-CREB cascade and induction of BDNF could mediate long-term adaptations of neuronal function in response to antidepressant treatments.